1. Field of the Invention
The present invention is generally related to surface plasmon resonance based devices. More particularly, the present invention is related to surface plasmon resonance devices used in a photolithographic system.
2. Related Art
Surface plasmon resonance (SPR) is the oscillation of the plasma of free electrons that exists at a metal boundary. These oscillations are affected by the refractive index of the material adjacent the metal surface. Surface plasmon resonance may be achieved by using an evanescent wave that is generated when a p-polarized light beam is totally internally reflected at the boundary of a medium, e.g., glass, which has a high dielectric constant. A paper describing the technique has been published under the title “Surface plasmon resonance for gas detection and biosensing” by Lieberg, Nylander and Lundstrom in Sensors and Actuators, Vol. 4, page 299 (1983).
FIG. 1A shows a diagram of the conventional plasmon sensor equipment described in the Lieberg paper. An incident beam of light 1 is directed from a laser source (not shown) onto an internal portion of surface 2 of a glass body 3. A detector (not shown) monitors the internally reflected beam 4. Applied to the external portion of surface 2 of glass body 3 is a thin film of metal 5, for example gold or silver, and applied to the film 5 is a further thin film of material 6. A sample 7 is brought into contact with the film 6 to thus cause a reaction. If binding occurs, the refractive index of the film 6 will change, and this change can be detected and measured using surface plasmon resonance techniques.
Surface plasmon resonance can be experimentally observed by varying the angle of the incident beam 1 and monitoring the intensity of the internally reflected beam 4. At a certain angle of incidence, the parallel component of the light momentum will match with the dispersion for surface plasmons at the opposite surface 8 of the metal film 5. Provided that the thickness of metal film 5 is chosen correctly, there will be an electromagnetic coupling between the glass/metal interface at surface 2 and the metal/sample interface at surface 8, resulting in surface plasmon resonance, and thus attenuation in the reflected beam 4 at that particular angle of incidence. Thus, as the angle of incidence of incident beam 1 is varied, surface plasmon resonance is observed as a sharp dip in the intensity of the internally reflected beam 4 at a particular angle of incidence. The angle of incidence at which resonance occurs is affected by the refractive index of the material against the metal film 5, i.e. the film 6, and the angle of incidence corresponding to resonance is thus related to the refractive index of the sample. Increased sensitivity can be obtained by choosing an angle of incidence half way down the reflectance dip curve where the response is substantially linear, and then maintaining that angle of incidence fixed and observing changes in the intensity of the reflected beam 4 with time. This is illustrated in FIG. 1B.
As the angle of incidence is changed, either by moving the light source or rotating the glass body, or both, the point on surface 2 at which the incident beam 1 is incident moves. Because of inevitable variations in the metal film 5 and the film 6, the angle of incidence at which resonance occurs changes as the point of incidence of incident beam 1 moves, which, in turn, introduces a further variable factor into the measurement and thus makes comparison between the initial unbound state and the bound state of the film 6 less accurate.
Newly developed lithography machines with immersion have a fluid between the last lens of the projection optics (PO) and the wafer. Ultra pure water is used in such immersion lithography machines that utilize excimer lasers (that emit light at a wavelength of, e.g., 193 nm), and flows between the last lens element of the PO and the substrate (e.g., a wafer, a flat panel display, a printhead, or the like) in order to enlarge the depth of focus and to enable POs with a numerical aperture (NA) larger than 1. This enables the critical dimension of the semiconductor devices to be reduced. In order to avoid contamination of the projection optics and wafer, the water must be clean. In order to avoid shading effects during projection, the water needs to be free of particles and bubbles. Particles are also to be avoided to minimize the number of contaminants deposited on the wafer. The supply also needs to maintain a refractive index (n) of the fluid that is within a very small range. However, the refractive index n of the immersion media (IM) can vary due to the introduction of contaminants. Out-of range variations in n will lead to variations in critical dimension and critical dimension uniformity that will reduce wafer yield.
There is a need in the art for an improved SPR sensor, as well as for apparatus and methods relating thereto.